Rubber composition

ABSTRACT

As a rubber composition having an elasticity further increased as compared with the conventional rubber composition while preventing the lowering of the fracture resistance, there is provided a rubber composition comprising a rubber component of at least one of natural rubber and synthetic diene rubbers and a phenolic resin compounded therein and represented by the following formula (I): 
                         
(wherein R 0  is a hydrogen atom, an alkyl group, a phenyl group or a methylol group, and each of R 1  and R 2  is an arylene group, an alkylene group having a carbon number of 2-10, an aralkylene group, a cycloalkenylene group or a cycloalkadienylene group, and n is 0-10).

TECHNICAL FIELD

This invention relates to a rubber composition, and more particularly toa rubber composition including a phenolic resin of a specific structuresuitable for use in a carcass member of a tire, a conveyor belt, a hoseand the like.

BACKGROUND ART

As a countermeasure for increasing an elasticity of a rubber, there havehitherto been proposed a method of increasing an amount of a filler, amethod of increasing an amount of sulfur to increase the number ofcrosslinking points, and the like. However, these methods have a problemthat the characteristics such as fracture resistance and the like areconsiderably deteriorated.

On the contrary, as a countermeasure for increasing the elasticity ofthe rubber while suppressing the lowering of the fracture resistance ofrubber, there are proposed a method of adding a non-modifiednovolac-type or resole-type phenolic resin, and a method of adding aphenolic resin modified with an unsaturated oil such as tall oil orcashew oil or an aromatic hydrocarbon such as xylene or mesitylene.These methods are widely used for increasing the elasticity whilesuppressing the lowering of the fracture resistance (see, for example,JP-A-5-98081 and JP-A-2001-226528).

However, performances required in the rubber become very severer inrecent years, and hence it is required to further increase theelasticity of the rubber while suppressing the lowering of the fractureresistance of the rubber. The aforementioned method of adding thenon-modified phenolic resin or the modified phenolic resin isinsufficient in the meaning that the elasticity of the rubber is furtherincreased while suppressing the lowering of the fracture resistance.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to provide a rubbercomposition having an elasticity higher than that of the conventionaltechnique while preventing the lowering of the fracture resistance.

The inventor has made various studies for achieving the above object andfound that the elasticity can be further increased by adding a phenolicresin of a specific structure to the rubber composition while preventingthe lowering of the fracture resistance.

That is, the rubber composition according to the invention ischaracterized by compounding a rubber component comprised of at leastone of natural rubber and synthetic diene rubbers with a phenolic resinrepresented by the following formula (I):

(wherein R⁰ is a hydrogen atom, an alkyl group, a phenyl group or amethylol group, and each of R¹ and R² is an arylene group, an alkylenegroup having a carbon number of 2-10, an aralkylene group, acycloalkenylene group or a cycloalkadienylene group, and n is 0-10).

In a preferable embodiment of the rubber composition according to theinvention, each of R¹ and R² in the formula (I) is a xylylene group or[1,1′-biphenyl]-4,4′-dimethylene group.

In another preferable embodiment of the rubber composition according tothe invention, R⁰ in the formula (I) is a hydrogen atom.

In the other preferable embodiment of the rubber composition accordingto the invention, the compounding amount of the phenolic resin of theformula (I) is 1-30 parts by mass, more preferably 1-10 parts by massper 100 parts by mass of the rubber component.

In a further preferable embodiment of the rubber composition accordingto the invention, a hardening agent as a methylene donor is contained inthe rubber composition at an amount corresponding to 1-30% by mass ofthe phenolic resin of the formula (I). In this case, the hardening agentis preferable to be hexamethylene tetramine.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail below. The rubber compositionaccording to the invention is obtained by compounding the rubbercomponent comprised of at least one of natural rubber and syntheticdiene rubbers with the phenolic resin represented by the formula (I).The non-modified phenolic resin used in the conventional rubbercomposition and represented by the following formula (II) is low in thedispersibility to the rubber component such as natural rubber andsynthetic diene rubber being low in the polarity because a distancebetween phenol group and phenol group as a polar functional group isshort. Therefore, a portion of unevenly distributing the phenolic resinis existent in the rubber composition, and such a portion constitutes astarting point of breakage to lower the fracture resistance.

On the other hand, in the conventionally modified phenolic resin, thephenol resin is polymerized and thereafter the resulting polymer ismodified, so that only a terminal of the molecule in the structure ofthe polymer is modified and a central portion thereof is the same as inthe non-modified phenolic resin. As a result, the compatibility with therubber component is improved in the terminal of the molecule, but thecompatibility in the central portion is low and hence satisfactoryrubber properties are not obtained.

On the contrary, the phenolic resin represented by the formula (I)improves the compatibilities of both the terminal and central portionwith the rubber component because phenol and xylylene are bonded atmolecular unit as shown by the following formula (III).

Particularly, in the phenolic resin of the formula (I), a plurality ofphenols are bonded through a bivalent group larger than methylene, sothat a distance between phenol group and phenol group as a polarfunctional group becomes longer than that of non-modified or modifiedphenolic resins conventionally used in the rubber composition.Therefore, the phenolic resin of the formula (I) is low in the polarityas compared with the non-modified and modified phenolic resins, so that{circle around (1)} the dispersibility in the rubber component isimproved and hence the elasticity of the rubber composition can belargely increased. Also, since the dispersibility in the rubbercomponent is good, a uniform portion as a starting point of breakage isnot existent in the rubber composition according to the invention, sothat {circle around (2)} the form after the hardening is stronger tostress and the lowering of the fracture resistance is suppressed atminimum.

As the rubber component constituting the rubber composition according tothe invention are mentioned natural rubber (NR); and synthetic dienerubbers such as polyisoprene rubber (IR), butyl rubber (IIR),polybutadiene rubber (BR), styrene-butadiene copolymer rubber (SBR) andthe like. These rubber components may be used alone or in a blend of twoor more.

The phenolic resin used in the rubber composition according to theinvention is represented by the formula (I). In the formula (I), thenumber of repeat units n is 0-10. Also, R⁰ is a hydrogen atom, an alkylgroup, a phenyl group or a methylol group. In this case, the alkyl groupincludes methyl group, ethyl group and the like. Among them, thehydrogen atom is preferable as R⁰ in view of the hardening reaction.

In the formula (I), each of R¹ and R² is an arylene group, an alkylenegroup having a carbon number of 2-10, an aralkylene group, acycloalkenylene group or a cycloalkadienylene group. Although R¹ and R²are methylene groups in the phenolic resin compounded in theconventional rubber composition for the purpose of increasing theelasticity, R1 and R2 of the phenolic resin used in the invention arebivalent groups larger than methylene group, so that the distancebetween the phenol groups becomes longer and the aforementioned effectis developed. As the arylene group are mentioned phenylene group and thelike. As the alkylene group having a carbon number of 2-10 are mentionedethylene group, propyloene group and the like. As the aralkylene groupare mentioned xylylene group, [1,1′-biphenyl]-4,4′-dimethylene group andthe like. As the cycloalkenylene group are mentioned cyclohexylenegroup, dicyclodecylene group, tricyclodecylene group and the like. Asthe cycloalkadienylene group are mentioned cyclopentadienylene group andthe like. Among them, xylylene group or [1,1′-biphenyl]-4,4′-dimethylenegroup is preferable as R¹ and R².

As the phenolic resin of the formula (I) compounded in the rubbercomposition according to the invention can be used commerciallyavailable phenolic resins, for example, MEH-7800, MEH-7851 and the likemade by Meiwa Kasei Co., Ltd.

The compounding amount of the phenolic resin of the formula (I) is 1-30parts by mass, preferably 1-10 parts by mass per 100 parts by mass ofthe rubber component. When the amount is less than 1 part by mass, thehardening performance is insufficient, while when it exceeds 30 parts bymass, the flexibility as the rubber is damaged.

The rubber composition according to the invention is preferable tofurther contain a hardening agent as a methylene donor. However, when R0is methylol group, the phenolic resin itself is self-hardening, andhence the hardening agent is useless. As the hardening agent arementioned hexamethylene tetramine, hexamethyl methylol melamine and thelike. In this case, the compounding amount of hardening agent is 1-30%by mass of the phenolic resin of the formula (I). When the amount isless than 1% by mass, the hardening of the phenolic resin is notsufficiently promoted, while when it exceeds 30% by mass, thecrosslinking system of rubber is badly affected.

In addition to the rubber component, phenolic resin and hardening agentas mentioned above, the rubber composition according to the inventionmay be properly compounded with additives usually used in the rubberindustry such as fillers, softening agents, anti-oxidants, vulcanizingagents, vulcanization accelerators and the like.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES

A rubber composition is prepared according to a compounding recipe shownin Table 1 and vulcanized at 145° C. for 30 minutes to prepare a sample,which is subjected to a hardness test, a tensile test and a dynamicviscoelastic test by the following methods, respectively. The resultsare represented by an index on the basis that Comparative Example 1 is100 and shown in Table 1. Moreover, the phenolic resin used in Example 1is represented by the formula (III), and the phenolic resin used inExample 2 is represented by the following formula (IV):

(Measurement of Hardness)

With respect to the vulcanized rubber compositions, JIS A hardness ismeasured according to JIS K6253.

(Tensile Test)

A dumbbell type JIS No. 3 sample is prepared from the vulcanized rubbercomposition and subjected to a tensile test at 25° C. according to JISK6251 to measure elongation at break, tensile strength and tensilestress at an elongation of 50%.

(Dynamic Viscoelastic Test)

With respect to the vulcanized rubber compositions, the dynamic storagemodulus of elasticity (E′) and loss tangent (tan δ) are measured under1% strain and a measuring temperature of 25° C. using a spectrometermade by Toyo Seiki Co., Ltd.

TABLE 1 Comparative Comparative Example Example Example 1 Example 2 1 2Natural rubber parts by 100 100 100 100 Carbon black HAF mass 50 50 5050 Aromatic oil 5 5 5 5 Stearic acid 3 3 3 3 Zinc oxide 4 4 4 4Non-modified phenolic resin *1 10 — — — Phenolic resin (modified withcashew) *2 — 10 — — Phenolic resin (xylylene type) *3 — — 10 — Phenolicresin (biphenylene type) *4 — — — 10 Hexamethylene tetramine 1 1 1 1Antioxidant 6PPD *5 1 1 1 1 Vulcanization accelerator TBBS *6 1.5 1.51.5 1.5 Sulfur 2 2 2 2 Hardness index 100 105 105 110 Elongation atbreak 100 89 98 97 Tensile strength 100 83 91 97 Tensile stress at 50%elongation 100 110 128 159 Dynamic storage elasticity (E′) 100 121 135190 Loss tangent (tan δ) 100 104 100 107 *1: PR-50731 made by SumitomoBakelite Co., Ltd. softening point 95° C. *2: PR-12686 made by SumitomoBakelite Co., Ltd. softening point 75° C. *3: MEH-7800-3H made by MeiwaKasei Co., Ltd. number of repeat units (n) = 0-7, softening point 102°C. *4: MEH-7851-4H made by Meiwa Kasei Co., Ltd. number of repeat units(n) = 0-7, softening point 130° C. *5:N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylene diamine *6:N-t-butyl-2-benzothiazole sulfenamide

In Comparative Example 2, the drynamic storage modulus and loss tangentare raised to increase the elasticity by replacing the non-modifiedphenolic resin of Comparative Example 1 with the modified phenolicresin, but the elongation at break and the tensile strength are loweredto largely lower the fracture resistance.

On the other hand, in Examples 1 and 2 using the xylylene type orbiphenylene type phenolic resin, the lowering of the elongation at breakand tensile strength is suppressed and the dynamic storage modulus ofelasticity and loss tangent are improved while preventing the loweringof the fracture resistance, whereby the elasticity can be considerablyincreased.

INDUSTRIAL APPLICABILITY

According to the invention, there can be provided rubber compositionshaving a considerably high elasticity while preventing the lowering ofthe fracture resistance by compounding the phenolic resin of thespecific structure to the rubber component.

1. A rubber composition characterized by compounding a rubber componentconsisting of natural rubber and/or polyisoprene rubber with a phenolicresin represented by the following formula (I):

(wherein R⁰ is a hydrogen atom, an alkyl group, a phenyl group or amethylol group, and each of R¹ and R² is an arylene group, an aralkylenegroup, a cycloalkenylene group or a cycloalkadienylene group, and n is0-10).
 2. A rubber composition according to claim 1, wherein each of R¹and R² in the formula (I) is a xylylene group.
 3. A rubber compositionaccording to claim 1, wherein each of R¹ and R² in the formula (I) is[1,1′-biphenyl]-4,4′-dimethylene group.
 4. A rubber compositionaccording to claim 1, wherein R⁰ in the formula (I) is a hydrogen atom.5. A rubber composition according to claim 1, wherein the compoundingamount of the phenolic resin of the formula (I) is 1-30 parts by massper 100 parts by mass of the rubber component.
 6. A rubber compositionaccording to claim 5, wherein the compounding amount of the phenolicresin of the formula (I) is 1-10 parts by mass per 100 parts by mass ofthe rubber component.
 7. A rubber composition according to claim 1,wherein a hardening agent as a methylene donor is contained in therubber composition at an amount corresponding to 1-30% by mass of thephenolic resin of the formula (I).
 8. A rubber composition according toclaim 7, wherein the hardening agent is hexamethylene tetramine.
 9. Arubber composition according to claim 2, wherein R⁰ in the formula (I)is a hydrogen atom.
 10. A rubber composition according to claim 3,wherein R⁰ in the formula (I) is a hydrogen atom.
 11. A rubbercomposition according to claim 2, wherein the compounding amount of thephenolic resin of the formula (I) is 1-30 parts by mass per 100 parts bymass of the rubber component.
 12. A rubber composition according toclaim 3, wherein the compounding amount of the phenolic resin of theformula (I) is 1-30 parts by mass per 100 parts by mass of the rubbercomponent.
 13. A rubber composition according to claim 4, wherein thecompounding amount of the phenolic resin of the formula (I) is 1-30parts by mass per 100 parts by mass of the rubber component.
 14. Arubber composition according to claim 9, wherein the compounding amountof the phenolic resin of the formula (I) is 1-30 parts by mass per 100parts by mass of the rubber component.
 15. A rubber compositionaccording to claim 10, wherein the compounding amount of the phenolicresin of the formula (I) is 1-30 parts by mass per 100 parts by mass ofthe rubber component.